Apparatus for varying the cross-sectional area of the throat of a venturi tube



' May 12., 1970 R. s. DUBROVSKY EI'AL 3,51

APPARATUS FOR VARYING THE CROSSSECTIONAL AREA OF THE THROAT OF A VENTURI TUBE Filed July 9, 1968 E5 i I I. 32 I 33 34:; O I

\a i: i \b I 5 \c if U T w r A I To Compressed Air J To Vacuum Source lNVE/VTORS' Plea/d0 S. Dubrovsky By James P. Toma/2y United States Patent US. Cl. 137-487 5 Claims ABSTRACT OF THE DISCLOSURE This method and apparatus for varying the cross-sectional area of the throat of a venturi tube involves at least partially evacuating the space enclosed between a flexible throat of a venturi tube and a rigid shell encompassing the throat and forming an airtight seal with the venturi tube. Evacuation is effected to the degree necessary to maintain a constant pressure differential between the intake and exhaust of the venturi tube at varying process gas flow.

This method and apparatus for varying the cross-sectional area of the throat of a venturi tube involves evacuating the space enclosed between a flexible throat of a venturi tube and a rigid shell encompassing the throat and forming an airtight seal with the venturi tube. More particularly, evacuation is effected to reduce to the necessary extent the pressure in the space enclosed by the shell and the venturi tube to prevent the central portion of the flexible throat from being drawn radially inward by a gas stream, under negative pressure, passing through the venturi tube.

There are several conventional arrangements for varying the cross-sectional area of a venturi throat. One of these is a mechanical arrangement which utilizes a mechanical pincer means to position the flexible walls of a venturi tube radially inward or outward in order to maintain a constant pressure drop between the intake and the exhaust of the venturi tube. This system can be operated by changes in the pressure differential across the venturi tube. When the process gas flow rate through the venturi tube is decreased at the intake, a smaller pressure differential is momentarily created across the venturi tube. Accordingly, the mechanical pincers constrict the walls of the venturi throat radially inward, thereby decreasing the cross-sectional area and increasing the pressure drop across the tube. If the reduction in the throat cross-section area is sufficient, the pressure differential returns to its former value. Conversely, when process gas flow at the intake increases, the mechanical pincers are operated to expand the walls of the throat, thus increasing the crosssectional area of the throat. This arrangement has the disadvantage in that the system has a slow response time in making adjustments, and the adjustments made are inaccurate when a compressible gas stream is passed through the venturi tube.

Another device which has been used to vary the crosssectional area of a flexible venturi throat is an inflatable material lining the inside of the venturi throat. When the gas flow decreases at the venturi intake, the material lining the throat is inflated, thus decreasing the eflective crosssectional area of the throat, thereby maintaining the pressure differential desired across the venturi tube. This arrangement is disadvantageous in that while it functions properly when used with all incompressible fluids, it will not accomplish its purpose when a compressible gas passes through the venturi tube and when the pressure of the ice compressible gas stream at the venturi throat is less than the ambient pressure surrounding the throat.

The reason for some of the difiiculties encompassed by the conventional systems in maintaining a given pressure differential in a passing gas stream across a venturi tube by varying the cross-sectional area of the venturi tube, is due to the variation in pressure within the venturi tube. In the flow of a compressible gas through a venturi tube, the gas passes from a relatively high pressure and low velocity at the intake to a relatively low pressure and high velocity at the throat. From this it can be seen that the pressure of the gas at the throat of the venturi tube may often drop below the ambient air pressure surrounding the venturi tube. Also in many process systems, the venturi throat is under a negative pressure by virtue of the location of the process gas flow. In either case, where the throat of a venturi tube is flexible, the ambient pressure unrestrained, will force the walls of the throat radially inward when the pressure inside the throat is less than surrounding ambient pressure. If this is allowed to happen, the rate of flow and the intake pressure will determine the cross-sectional area of the throat, and consequently the pressure ditferential between the intake and the exhaust of the venturi tube. In such a situation the control that is required to maintain the proper pressure differential across the venturi tube is not a means for forcing the walls of the venturi throat inward to form a smaller crosssection, as has been attempted, but rather a means for forcing the walls of a venturi throat outward to form a larger cross-sectional area. This is accomplished using the present invention.

It may be considered a principal object of the present invention to provide the apparatus necessary to effect the variation of the cross-sectional area of the throat of a venturi tube in order to maintain constant a desired pressure differential in a compressible gas stream between the intake and the exhaust of a venturi tube at varying process gas flow rates. It is a further object to provide a system which reacts to flow changes in the venturi tube more quickly and accurately than do conventional means, in order to maintain a constant pressure differential across a venturi tube. Another objective is to effect the accurate maintenance of a given pressure differential across a venturi tube under any prevailing ambient pressure conditions.

The applications of such an invention occur in a variety of areas. One principal use of this invention is in a venturi scrubber systems for cleaning particle laden gases. In this application a gas stream containing dust particles and a water mist is passed through a venturi tube to assist in the agglomeration of the particles. In this instance the desired pressure differential must be maintained constant across the venturi tube despite variations of input pressure or flow rate. This application is practical with any system in which undesired particles are to be removed from a gas stream. Another application is to fuel injection systems in internal combustion engines. Particular use of this application is found in astronautical, airborne, terrestrial and nautical vehicles and vessels employing internal combustion engines for propulsion. Other applications include such things as natural gas heating devices, suction cleaning devices, and gas fuel torches.

In a broad aspect this invention provides in combination with a venturi tube having an intake, a throat, and an exhaust, the improvement comprising a flexible throat exhaust having an inlet end sealed airtight to the intake of the venturi tube, having an outlet end sealed airtight to the exhaust of the venturi tube, and having a flexible, radially movable central portion, a rigid shell encompassing said throat and forming an airtight intersection with said venturi tube at both ends of said throat; a vacuum source; a valving means connected to said vacuum source and positionable to allow air to be drawn to said vacuum source to a varying extent from the space bounded by said shell and said venturi tube; a connector tube connecting said valving means to the space bounded by said shell and said venturi tube; a pressure differential measuring device connected both to the intake and the exhaust of said venturi tube; and a pressure differential transmitter connected to said pressure differential measuring device and to said valving means, whereby said pressure differential transmitter positions said valving means in response to pressure differential changes in order to maintain a predetermined pressure differential between the intake and exhaust of the venturi tube; a connection line connecting said valving means to said pressure differential transmitter; and a connection means connecting said pressure differential transmitter to said pressure differential measuring device.

In another aspect this invention is a method of varying cross-sectional area of a flexible venturi tube throat having a flexible radially movable central portion, said throat being encompassed by a rigid shell which forms an airtight intersection with the venturi tube at both ends of said throat, comprising the steps of: measuring pressure diflferential of a compressible gas stream across the venturi tube; and evacuating the space enclosed between said venturi tube and said rigid shell in response to and in proportion to changes in pressure differential across said venturi tube.

The variation in cross-sectional area of the venturi tube in response to pressure differential variations across the venturi tube acts to maintain a desired pressure differential at varying process gas flow rates. The principles of fluid dynamics can be used to show that in a gas flow through a venturi tube, the pressure varies inversely with the velocity. Thus, since the velocity at the throat of the venturi tube will always be greater than the velocity at the intake, the pressure of a compressible gas stream of the venturi tube at the throat will always be less than the pressure at the intake. Where the gas flow is initially at low pressure, that is at an intake pressure only slightly above atmospheric pressure, the pressure at the throat will often fall below atmospheric pressure. The partial vacuum thus produced, if unrestricted, will result in the central portion of the flexible throat being forced radially inward toward the center line of flow through the venturi tube by the ambient atmospheric pressure surrounding the flexible throat. Subsequent increases in gas flow at the intake tend to further reduce pressure at the throat and increase the resultant pressure drop from the intake to the exhaust. An unrestricted atmospheric pressure would thus tend to close the flexible throat even more. Subsequent decreases in gas flow at the intake tend to increase pressure at the throat and decrease the resultant pressure drop from the intake to the axhaust. The increased pressure at the throat, in this case, tends to force the central portion of the flexible throat radially outward somewhat. In either case, the pressure drop from the intake to the exhaust changes. When it is desired to maintain the pressure from the intake to the exhaust at a predetermined level during variations in flow conditions, the expansion and contraction of the throat can be controlled to accomplish this end by the use of the apparatus of this invention.

The physical construction and operation of the valving means and the pressure differential transmitter of this invention may take a variety of forms. Either of these arrangements may be operated in a number of ways including, but not limited to electric, spring biased, and pneumatic operation, or any combination thereof. Any compatible constructions of these devices may be used with each other. The preferred embodiments of this invention utilize either a pressure differential transmitter which emits an electrical signal to position the valving means, or a pressure differential transmitter which emits a pneumatic signal to position the valving means.

The venturi tube normally is constructed symmetrical about a linear axis. However, the shape of the venturi tube may be modified for some applications, and the axis about which the component parts of the venturi tube are mounted may have a curvature of a more unconventional nature.

The various features of the preferred embodiment of the present invention are illustrated in the accompanying drawing which is a vertical section of the invention.

Referring now to the drawing, venturi tube 23 is comprised of intake 2, a flexible rubber throat 1, and exhaust 3. While throat 1 as illustrated is constructed of rubber, it may be constructed of any organic elastic material. Intake 2, throat 1, and exhaust 3 are shown arranged on a common axis. Flexible throat I has its inlet end 32 sealed airtight around the entire perimeter of the juncture with the adjacent portion of intake section 2 of venturi tube 23, while an outlet end 34 is sealed airtight around the entire perimeter of the adjacent portion of exhaust section 3 of venturi tube 23. The central portion 33 of the throat section shall be readily flexible and radially movable with respect to said axis of the entire unit. In the present embodiment, the pressure at point a, a point in the intake section 2, is transmitted through a connector leg 24 to a manometer 5. From point e, a point in the exhaust section 3, pressure is transmitted through a connector leg 25 to a manometer 5. Point b is designated as being within the narrowest portion of flexible throat I.

A shell 4 constructed of steel or other material more rigid than throat 1, is shown encompassing throat 1 and forming an airtight intersection with the venturi tube 23 at both ends of throat I. That is, shell 4 circumferentially intersects and becomes sealed with intake section 2 and exhaust section 3. A connector tube 9 connects the zone within shell 4 to valving means 11. Valving means 11 is, in turn, connected to a vacuum source (not shown) by line 12. Valving means 11 is positionable to allow air to be drawn to said vacuum source alternatively and to varying degrees through connector tube 9 from the space bounded by shell 4 and venturi tube 23, and from the atmosphere through line 10. In the preferred embodiment, valving means 11 is comprised of an enclosing retainer wall 27 in which there is an air passage to line 12. This air passage remains open during all phases of the operation of the invention. Also present is an air passage to line 10 which leads to the atmosphere, and an air passage to connector tube 9, which leads to shell 4. There is also an air passage block 13 which is pivoted about pivot means 26 so as to be positionable to block the air passage to line 10, completely or partially. Valving means 11 is further comprised of an elongated inflatable tube or bag 14, crimped to and attached to air passage block 13, a spring means 16 which tends to pull air passage block 13 away from line 10, and a connection line 15 connected to an inflatable bag 14 and leading to a pressure differential transmitter 17. When air pressure is transmitted from pressure differential transmitter 17 through connection line 15, bag 14 is inflated and tends to straighten out in its direction of elongation, from its ciimped position against air passage block 13, thereby forcing air passage block 13 to pivot about pivot means 26 away from connection tube 9 and in front of line 10. When air pressure is not transmitted from pressure differential transmitter 17 to connection line 15, inflated bag 14 loses air pressure back through connection line 15, and bag 14 resumes its uninflated position against block 13, as spring 16 pivots block 13 about pivot means 26 to draw block 13 away from line 10.

Pressure differential transmitter 17 emits a pneumatic signal through tube 15 to position valving means 11. Pressure differential transmitter 17 is operated independently by a compressed air supply (not shown), and is comprised of a rigid, enclosing wall 30 with an opening to connection line 15 and with an air pressure relief valve 29. An air supply tube 22, connected to the compressed air supply, enters through an opening in wall 30 and forms an airtight seal with wall 30, as does connection line 15. A portion of the air supply tube 22 extends into pressure differential transmitter 17, and in this portion of tube 22 is positioned an electromagnetic relay means 18, which, when operated, draws spring steel bar 19 to it. Spring steel bar 19 is rigidly fastened to air supply tube 22 at weld 28. To spring steel bar 19 is attached a needle valve 20 which seats in valve seat 21, thereby forming the end of air supply tube 22. Electromagnetic relay 18 is strong enough to overcome pressure from the compressed air supplied through tube 22, and to thereby pull spring steel bar 19 upward when operated by an electric current supplied by a battery through insulated electric wires 7 and 8 Which enter and are sealed to wall 30 and air supply tube 22. When electromagnetic relay 18 is operated, spring steel bar 19 is drawn upward, needle valve 20 is unseated from valve seat 21, and compressed air is allowed to enter the space enclosed by wall 30 and is thereby transmitted to connection line 15 to operate valving means 11. When the electromagnetic relay is no longer operated, spring steel bar 19 returns to its normal position, thus seating needle valve 20 in valve seat 21, thereby precluding further pressure from being transmitted to valve 11. The existing pressure in connection line 15 is relieved through pressure relief valve 29 which allows air to slowly escape to the atmosphere, when needle valve 20 is seated, through pressure relief valve 29. Pressure relief valve 29 is of insufficient size to materially affect the pressure being transmitted to connection line 15 when needle valve 20 is unseated.

Electromagnetic relay 18 is operated by a battery when contacts d and e are closed in a leg of manometer connected to connector leg 25. Contacts at and e are closed when the pressure differential between intake section 2 and exhaust section 3 of venturi tube 23 rises sufficiently to allow the mercury 6 in manometer 5 to rise to the level of contacts d and e in the leg connected to connector leg 25. The leg leading to connector leg 25 has a number of paired copper wires extending through its wall at various levels. Contact a, which is connected to wire 31, is attached to one of the pair of copper wires at the level desired. Pressure differential transmitter 17 is connected to the pressure differential measuring device, manometer 5, by a connection means comprising contacts d and e, and wires 7, 8, and 31. Contact 2, which is connected to wire 8, is attached to the other of the pair of copper wires at the level desired. When mercury 6 rises to the level of the pair of copper wires chosen, the electrical circuit is completed from the battery, through wire 7 to electromagnetic relay 18, through wire 8 to contact e, through the pair of copper wires, as connected by mercury 6, to contact d, and through wire 31 back to the battery.

When the pressure at point a increases, the tendency, as heretofore explained, is for the pressure drop between a and c to increase and for the pressure at b to decrease, thus contracting the opening of throat 1. This effect is prevented by the present invention. When the pressure increases between a and c, the level of mercury 6 rises in the leg of manometer 5 leading to connector leg 25, thus closing the contacts of the electric circuit and operating electromagnetic relay 18. Electromagnetic relay 18 thus draws spring steel bar 19 upward, thereby unseating needle valve 20 and allowing pressure from a compressed air supply to enter pressure differential transmitter 17 from an air supply tube 22, from where it is transmitted through line 15 to valving means 11. The compressed air pressure in connection line 15 inflates bag 14, thereby forcing air passage block 13 in front of line 10. Thus, the vacuum connected to valving means 11 draws air from the space encompassed by rigid shell 4, thereby equalizing pressure between that space and the pressure in the valve throat at b. With the pressure equalized, there is no longer a tendency for flexible throat 1 to contract. When the space encompassed by rigid shell 4 is sufficiently evacuated, the pressure drop from a to 0 will again reach the desired level and the electric contact through mercury 6 will be broken. When this occurs, the result in the apparatus depicted is the same as when pressure at point a decreases.

When the pressure, and consequently the velocity, at point a drop, there is a tendency for pressure between a and c to decrease and for pressure at b to increase, thus widening the opening of throat 1. This effect is prevented by this invention. When pressure decreases between points a and c, the level of mercury 6 falls in the leg of manometer 5 leading to connector leg 25, thus opening the contacts d and e of the electric circuit operating electromagnetic'relay 18. Electromagnetic relay 18 thus releases spring steel bar 19 which returns to its normal position, thereby seating needle valve 20 and precluding air pressure from a compressed air supply to enter pressure differential transmitter 17 through air supply tube 22. The compressed air pressure which has already inflated bag 14 is relieved by air escaping through pressure relief valve 29, and the spring means 16 draws air passage block 13 away from line 10. Thus, the vacuum connected to valving means 11 draws air to a large extent from the atmosphere through line 10, rather than from the space encompassed by rigid shell 4. This increase in the pressure inside the space enclosed between shell 4, intake section 2, exhaust section 3 and flexible rubber throat 1 forces the central portion 33 of rubber throat I radially inward, thereby increasing the pressure drop from a to 0 until this pressure drop reaches the desired level. If the pressure differential between a and 0 increases too much, the result in the apparatus depicted is the same as when the pressure at point a increases.

The present invention can be further illustrated by the following example.

EXAMPLE I A flexible throat of a venturi tube having a varying cross-sectional area and having a radially movable central portion, is encompassed by a rigid shell which forms an airtight intersection with the venturi tube on both sides of the throat.

The nominal cross-sectional area of the throat is one quarter that of both the intake and the exhaust, air passes through the venturi tube at a velocity of approximately 200 feet per second, the density of this air being .090 pound per cubic foot. Under these conditions, the pressure drop between the intake and exhaust of the venturi tube is about 15 inches of water, and the throat pressure is less than the ambient atmospheric pressure. When the intake process gas flow decreases with an attending decrease in velocity, there is a tendency for the pressure differential between the intake and the exhaust to drop also, and for the preSsure at the throat to increase. This tendency is overcome by measuring the pressure differen tial across the venturi tube, and reducing the degree of evacuation of the space enclosed by the rigid shell and the venturi tube in response to and in proportion to the pressure differential decrease across the venturi tube, thereby allowing pressure within the enclosed space to remain higher as a result of partial atmospheric pressure, thus preventing the throat from expanding radially outward.

If the gas flow rate again increases the pressure drop between the intake and exhaust tends to increase and the throat tends to close. However, further evacuation of the space around the throat and enclosed by the rigid shell in response to and in proportion to the increase in pressure differential across the venturi tube prevents the throat from so contracting and thus maintains the pressure drop between the intake and the exhaust.

The preferred embodiment, as illustrated in the drawing and example used, shall not be considered limiting as to the sizes and types of rigid shells, vacuum sources,

positionable valves, pressure differential measuring devices, and pressure difierential transmitters. Neither shall the component parts of these elements or the manner of internal operation of each of these units be considered limited. The size and relative dimensions of the venturi tube, and the operating velocities and pressures used likewise are not to be considered limiting.

We claim as our invention:

1. In combination with a venturi tube having an intake,

a throat, and an exhaust, the improvement comprising:

(a) A flexible throat having an inlet end sealed airtight to the intake of the venturi tube, having an outlet end sealed airtight to the exhaust of the venturi tube, and having a flexible radially movable central portion;

(b) a rigid shell encompassing said throat and forming an airtight intersection with said venturi tube on both sides of said throat;

(c) a vacuum source;

(d) a valving means connected to said vacuum source and positionable to allow air to be drawn to said vacuum source to a varying extent from the space bounded by said shell and said venturi tube;

(e) a connector tube connecting said valving means to the space bounded by said shell and said venturi tube;

(f) a pressure differential measuring device connected both to the intake and the exhaust of said venturi tube; and

(g) a pressure differential transmitter connected to said pressure differential measuring device and to said valving means, whereby said pressure difierential transmitter positions said valving means in response to pressure diiferential changes in order to maintain a pre-determined pressure ditferential between the intake and exhaust of the venturi tube;

(h) a connection line connecting said valving means to said pressure differential transmitter; and

(i) a connection means connecting said pressure differential transmitter to said pressure differential measuring device.

2. The apparatus of claim 1 further characterized in that said flexible throat is constructed of rubber and said shell is constructed of steel.

3. The apparatus of claim 1 further characterized in that said pressure differential transmitter emits an eleclrical signal to position said valving means.

4. The apparatus of claim 1 further characterized in that said pressure differential transmitter emits a pneumatic signal to position said valving means.

5. The apparatus of claim 1 further characterized in that said valving means is positionable to allow air to be drawn to said vacuum source alternatively and to varying degrees from said space bounded by said shell and said venturi tube and from the atmosphere.

References Cited UNITED STATES PATENTS HENRY T. KLINKSIEK, Primary Examiner R. I. MILLER, Assistant Examiner U.S. Cl. X.R. 

